In a communication system in which the received signal is perturbed by noise, interference, and fading in the radio channel, errors occasionally occur. To combat this, some systems utilize forward error correction (FEC) schemes, like convolutional coding or Turbo coding, where redundant bits are added into the original data stream and transmitted over the communication link. Using this redundant information, the receiver can correct errors that have occurred. Normally the performance of FEC schemes is best when the errors are evenly distributed in the received data stream and have equal probability of occurring. However, this is not always the case in practice: errors occur often in bursts. For example, fading causes rapid variation in the attenuation of a radio channel when a receiver is moving, and the received power can vary by as much as 25 dB over a period as small as 10 ms at 30 km/h. The probability of error is very high for bits that are transmitted in “bad channel conditions” (for example, when the received power is smaller than −10 dB), but the probability of error in receiving other bits is small.
To address fading, interleaving is often used: the FEC-coded bits are “mixed” (interleave) before transmission in such a way that the probability of transmission during temporarily bad channel conditions is more evenly distributed in the FEC-coded data stream. A common method for interleaving is to use a so-called block interleaver: data are written into an interleaver matrix row-by-row and read out column-by-column. Block interleaving is useful when using simple QPSK modulation, but causes problems when using higher-order modulation schemes like 16-QAM. In higher-order schemes, the modulation itself causes differences in the error-probability between different bits. For example, when 16-QAM is used together with Gray mapping, the interleaved bits are mapped into reliable (“R”) and unreliable (“U”) positions as R-R-U-U-R-R-, and so on. This can lead to the situation in which the FEC-coded data stream is mapped into reliable and unreliable positions as clusters, or bursts, and the interleaver actually worsens the problem, i.e. it can group together (or cluster) bits having a high probability of error.
The inventors are unaware of the prior art having directly addressed the problem of clustering caused by interleaving when using a higher-order modulation scheme. However the inventors are aware of a suggestion of separately interleaving data bits (systematic bits) and redundancy bits (parity bits) and of then mapping as many of the systematic bits into reliable positions, which is an altogether different procedure than that provided by the invention.
Thus, to address clustering in case of interleaving when using higher order modulation schemes, what is needed is a modification of the prior art block interleaver that avoids clustering but leaves unchanged advantageous properties of the block interleaver.